1. FIELD OF THE INVENTION
The invention relates to a two stage dilute acid prehydrolysis of biomass for solubilization of hemicellulosic sugars and a pretreatment for enzymatic hydrolysis of cellulose. In particular, the invention pertains to a two-stage dilute acid prehydrolysis treatment of a feedstock of hemicellulosic material comprising xylan that is slow hydrolyzable and xylan that is fast hydrolyzable under low temperature conditions to hydrolyze said fast hydrolyzable xylan to xylose; removing said xylose and leaving a feedstock residue containing said slow hydrolyzable xylan; treating said residue containing said slow hydrolyzable xylan with a dilute organic or inorganic acid under temperature conditions higher than said low temperature conditions to hydrolyze said slow hydrolyzable xylan to xylose, and removing said xylose.
2. DESCRIPTION OF THE PRIOR ART
U.S. Pat. No. 4,072,538 to Fahn et al. is directed to a process for the two-stage decomposition of hemicellulose to xylan containing natural products for the purpose of obtaining xylose, wherein the starting material is a basic medium and the residue is treated with an acid treatment, and the two stages are carried in the same reaction vessel.
U.S Pat. No. 4,105,647 to Buckl et al. employs a method for the two-stage digestion of natural products containing xylan in order to obtain xylose, wherein a vegetable material is treated with a basic substance and the residue is treated with an acid. The process uses two stages and is done at temperatures of from 50 to about 60 degrees celsius.
In U.S. Pat. No. 3,990,904 to Friese et al., xylose is prepared from oat husks by hydrolyzing oat husks with solutions of alkali metal hydroxide to remove acetic acid and then hydrolyzing the oat husks with a mineral acid to provide a solid residue containing lignin and xylose.
U.S. Pat. No. 3,954,497 to Friese is directed to a process for the hydrolysis of deciduous wood, wherein the hydrolysis is carried out in a first stage with an alkali metal hydroxide solution and in a second stage with a mineral acid. The resulting product is D-xylose.
The factor in common in all four of the foregoing patents is the use of two-stage treatments of biomass for the production of xylose; however, the first treatment is with an alkaline solution and the second treatment step is an acid hydrolysis step.
U.S. Pat. No. 4,168,988 to Riehm et al. pertains to a process for the winning of xylose, by hydrolysis of residues of the annuals. Xylose is produced from annuals by extracting substances from the annuals with an acid solution, then pressing, moistening with an acid solution, hydrolyzing by increasing the temperature, terminating the hydrolysis by dropping the temperature, extracting with water and purifying. However, while this is a two-stage process in which biomass is first washed with dilute acid and then hydrolyzed with dilute acid, the washing step is for purposes of removing cations, water soluble sugars and other extractives, and hydrolyzes only arabinose and other easy to hydrolyze linkages. The xylan bonds are not hydrolyzed during the first step, because this step is for the purpose of removing impurities from the xylose solution produced during the second, single-stage step.
U.S. Pat. No. 4,029,515 to Kiminki et al. is directed to a two-stage acid hydrolysis process, wherein xylose produced in the first stage is simultaneously converted to furfural.
In biomass materials, cellulose and hemicellulose are the two most abundant and renewable raw organic compounds, and together they compose about 70 percent of the entire world's plant biomass on a dry weight basis. These raw materials are widely available in waste from agricultural, forest, vegetable, and food process sources and the efficient recycling of these wastes to useful products, such as ethanol, would help reduce disposal problems as well as provide an abundant and cheap source of fuel.
Unlike cellulose, hemicellulose is readily and easily converted to its various hydrolysate by-products by mild acid hydrolysis or enzymatic hydrolysis treatment and the resultant byproducts include various pentoses (xylose and arabinose being the main derivatives), hexoses (mannose and galactose), and sugar acids. By far, D-xylose is the major hemicellulose hydrolysate and constitutes approximately 60 percent of the total hydrolysates produced therefrom.
However, under conventional processes, the xylose being formed by hydrolysis of xylan is also being continuously converted to furfural and other undesirable by-products of sugar decomposition, which are toxic to yeast and not convertible to ethanol. Thus, the yield of xylose achievable is limited, which in turn would decrease the ethanol yield upon fermentation.